1. Field of the Invention
The present invention relates to a deposition donor substrate which is used to form a layer including an organic compound, and to a deposition method using the substrate. The present invention also relates to a method for manufacturing a light emitting device having a layer including an organic compound as a light emitting layer.
2. Description of the Related Art
Light emitting elements including an organic compound as a luminous body, which have features such as thinness, lightness, high-speed responses, and DC drive at low voltage, are expected to be applied to next-generation flat panel displays. In particular, display devices in which light emitting elements are arranged in matrix are superior in viewing angle and visibility to conventional liquid crystal display devices.
It is said that a light emitting mechanism of a light emitting element emits light in the following manner: when voltage is applied between a pair of electrodes with an electroluminescent (hereinafter also referred to as EL) layer interposed therebetween, electrons injected from a cathode and holes injected from an anode are recombined at emission centers in the EL layer to form molecular excitons, and energy is released when the molecular excitons relax to the ground state. As excited states, a singlet excited state and a triplet excited state are known, and light emission is considered to be possible through either of the excited states.
An EL layer included in a light emitting element has at least a light emitting layer. An EL layer can also have a stacked-layer structure including a hole injecting layer, a hole transporting layer, an electron transporting layer, an electron injecting layer, and/or the like, in addition to a light emitting layer.
EL materials for forming EL layers are broadly classified into a low molecular (monomer) material and a high molecular (polymer) material. In general, a low molecular material is often deposited by an evaporation method and a high molecular material by an inkjet method, a spin coating method, or the like.
An evaporation apparatus used for an evaporation method has a substrate holder for mounting a substrate thereon, a crucible (or an evaporation boat) containing an EL material, i.e., an evaporation material, a heater for heating the EL material in the crucible, and a shutter for preventing a subliming EL material from being scattered. The EL material heated with the heater is sublimed and deposited onto the substrate. In order to achieve uniform deposition, a deposition target substrate needs to be rotated and the distance between the substrate and the crucible needs to be about 1 m even when the substrate has a size of 300 nm×360 mm. Thus, when the size of a substrate to be processed is increased, the size of an evaporation apparatus also needs to be increased more than that. Therefore, the size of a substrate onto which a film can be deposited by an evaporation method is practically limited.
When the evaporation method mentioned above is employed to manufacture a full-color display device with red, green, and blue light emitting elements, a shadow mask is provided in contact with a substrate between the substrate and an evaporation source and selective deposition for each color can be achieved through this mask.
However, a shadow mask which is used to manufacture a full-color display device is very thin because of the necessity for precise manufacture of its opening portion. Thus, if the size of a shadow mask is increased as the size of a substrate is increased, problems may arise in that a shadow mask bends and the size of an opening portion is changed. It is difficult to adopt a means for reinforcing a shadow mask in a region of the shadow mask corresponding to a pixel portion. Thus, in the case of manufacturing a large-area display region, it is also difficult to employ a reinforcing means.
Furthermore, miniaturization of each display pixel pitch is increasingly demanded with an increase in definition of a display device (with an increase in the number of pixels), and there is a trend toward even thinner shadow masks.
On the other hand, a method for directly forming an EL layer over a substrate by a wet method such as an inkjet method or a spin coating method can also be employed when the size of a substrate is increased; however, it is difficult to form a uniform film by this method. When a wet method is employed, it is necessary that a composition or a solution including an EL material is applied and then baked to remove a solvent. Thus, in the case of stacking layers including an EL material, an application step and a baking step need to be repeated, and it takes a very long time. In the case of stacking layers by a wet method such as an inkjet method, a layer has to be formed using a solvent in which a previously formed layer is not soluble, and there is only a limited choice of materials and stacked-layer structures to be used. If there is only a limited choice of materials or stacked-layer structures to be used, there is a significant limitation on the performance of a light emitting element (such as luminous efficiency or lifetime). Thus, a wet method may become a major obstacle to improvement in the performance of a light emitting device; for example, it may prevent the application of even a light emitting element with an excellent structure to a light emitting device.
Thus, a method for forming an EL layer of a light emitting element by laser thermal transfer has been proposed. Reference 1, for example, discloses a method for manufacturing an EL display panel, in which laser light passing through a slit is delivered to a donor substrate through a lens and an organic thin film layer provided over the donor substrate is transferred to a substrate.
[Reference 1] Japanese Published Patent Application No. 2002-222694